This invention relates to fiber optic pressure sensors, and more particularly to a fused fiber grating pressure sensor.
Sensors for the measurement of various physical parameters such as pressure and temperature often rely on the transmission of stain from an elastic structure (e.g., a diaphragm, bellows, etc.) to a sensing element. In a pressure sensor, the sensing element may be bonded to the elastic structure with a suitable adhesive.
It is also known that the attachment of the sensing element to the elastic structure can be a large source of error if the attachment is not highly stable. In the case of sensors which measure static, or very slowly changing parameters, the long term stability of the attachment to the structure is extremely important. A major source of such long term sensor instability is a phenomenon known as xe2x80x9ccreep,xe2x80x9d i.e., change in strain on the sensing element with no change in applied load on the elastic structure, which results in a DC shift or drift error in the sensor signal.
Certain types of fiber optic sensors for measuring static and/or quasi-static parameters require a highly stable, very low creep attachment of the optical fiber to the elastic structure. Various techniques exist for attaching the fiber to the structure to minimize creep, such as adhesives, bonds, epoxy, cements and/or solders. However, such attachment techniques may exhibit creep and/or hysteresis over time and/or high temperatures.
One example of a fiber optic based sensor is that described in U.S. Pat. No. 6,016,702 entitled xe2x80x9cHigh Sensitivity Fiber Optic Pressure Sensor for Use in Harsh Environmentsxe2x80x9d to Robert J. Maron, which is incorporated herein by reference in its entirety. In that case, an optical fiber is attached (by high temperature epoxy) to a compressible bellows at one location along the fiber and to a rigid structure at a second location along the fiber with a Bragg grating embedded within the fiber between these two attachment locations and in tension. As the bellows is compressed due to an external pressure change, the tension on the fiber grating is reduced, which changes the wavelength of light reflected by the grating. In that case, the attachment technique is an adhesive compound or bond, a mechanical attachment (shrink or press fit), or a welding or soldering of a metal coated fiber to a metallic rigid member. However, if the attachment of the fiber to the structure is not stable, the fiber may move (or creep) relative to the structure measurement inaccuracies.
In another example, a optical fiber Bragg grating pressure sensor secured in tension to a glass bubble by a UV cement is discussed in Xu, M. G., Beiger, H., Dakein, J. P., xe2x80x9cFibre Grating Pressure Sensor With Enhanced Sensitivity Using A Glass-Bubble Housing,xe2x80x9d Electronics Letters, 1996, Vol. 32, pp. 128-129.
However, as discussed hereinbefore, such attachment techniques may exhibit creep and/or hysteresis over time and/or high temperatures, or may be difficult or costly to manufacture.
Objects of the present invention include provision of a fiber optic pressure sensor with minimal creep.
According to the present invention, a pressure sensor is provided, comprising: an optical fiber, having at least one pressure reflective element embedded therein, the pressure reflective element having a pressure reflection wavelength; sensing means, fused to at least a portion of the fiber on opposite sides of the reflective element, for holding the fiber on opposite axial sides of the reflective element and for adjusting the strain on the fiber in response to external pressure; the reflective element having a predetermined amount of tension pre-strain; and the sensing means being strained due to a change in pressure, the strain causing a change in the pressure reflection wavelength, and the change in the pressure reflection wavelength being indicative of the change in pressure.
According further to the present invention, the sensing means comprises: a pair of end tubes being fused to at least a portion of the fiber on opposite sides of the reflective element; and an outer tube being fused to the end tubes. Additionally, the sensing means comprises a tube fused to at least a portion of the fiber on opposite axial sides of the reflective element. Further, according to the present invention, the reflective element is a Bragg grating.
The present invention provides an optical fiber and fiber grating put in tension within an all glass housing. The fiber is attached to the housing by fusing the fiber on opposite axial sides of the grating to the ends of the housing, thereby providing a creep resistant attachment of the fiber to the housing. Additionally, the outer walls (or cylinder or tube) of the housing are fused to the ends of the housing to provide an all glass enclosure which holds the optical fiber and grating in tension. In one further embodiment, pistons are used to provide a length amplification xcex94L/L. Still further, an outer tube may be fused to the fiber on opposite axial ends of the grating area adjacent to or a predetermined distance from the grating.
One or more gratings, fiber lasers, or a plurality of fibers may be used to sense the pressure. The fiber may be fused within the tube or partially within or on the outer surface of the tube.
The present invention substantially eliminates creep and other fiber attachment problems. Further, the invention provides low hystersis. Also, more than one grating may be embedded in the fiber in the sensor. Further, the invention may be used as an individual sensor or as a plurality of distributed multiplexed sensors. Also, the invention may be a feed-through design or a non-feed-through design.
The invention may be used in harsh environments, such as in oil and/or gas wells, engines, combustion chambers, and the like. In one embodiment, the invention may be an all glass sensor capable of operating at high pressures ( greater than 15 kpsi) and high temperatures ( greater than 150xc2x0 C.). The invention will also work equally well in other applications independent of the type of environment.
The foregoing and other objects, features, and advantages of the present invention will become more apparent in light of the following detailed description of exemplary embodiments thereof.